System and method for decoding commands based on command signals and operating state

ABSTRACT

A system and method for decoding command signals that includes a command decoder configured to generate internal control signals to perform an operation based on the command signals and an operating state. The same combination of command signals can request different commands depending on the operating state. A command is selected from a first set of operations according to the command signals when the memory system is in a first operating state and a command is selected from a second set of operations according to the command signals when the memory system is in a second operating state.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.12/820,877, filed Jun. 22, 2010, and issued as U.S. Pat. No. 8,205,055,which is a continuation of U.S. patent application Ser. No. 11/121,868,filed May 3, 2005, and issued as U.S. Pat. No. 7,757,061. Theseapplications and patents are incorporated herein by reference, in theirentirety, for any purpose.

TECHNICAL FIELD

The invention relates generally to command decoding for a memory system,and more particularly, to decoding command signals to perform operationsin the memory system based on the command signals and a operating stateof the memory system.

BACKGROUND OF THE INVENTION

Computer systems use memory devices, such as synchronous dynamic randomaccess memory (“SDRAM”) devices, to store instructions and data that areaccess by a processor. These memory devices are normally used as systemmemory in a computer system. In a typical computer system, the processorcommunicates with the system memory through a processor bus and a memorycontroller. The processor issues a memory request, which includes amemory command, such as a read command, and an address designating thelocation from which data or instructions are to be read. The memorycontroller uses the command and address to generate appropriate commandsignals as well as row and column addresses, which are applied to thesystem memory. In response to the commands and addresses, data istransferred between the system memory and the processor.

FIG. 1 is a functional block diagram of a conventional memory device100. The memory device 100 in FIG. 1 is an example of a double-data rate(DDR) SDRAM. The memory device 100 is referred to as a double-data-ratedevice because the data words DQ being transferred to and from thedevice are transferred at double the rate of a conventional SDRAM, whichtransfers data at a rate corresponding to the frequency of the appliedclock signal. The memory device 100 includes a control logic and commanddecoder 134 that receives a plurality of command and clock signals overa control bus CONT, typically from an external circuit such as a memorycontroller (not shown). The command signals include a chip select signalCS#, a write enable signal WE#, a column address strobe signal CAS#, anda row address strobe signal RAS#, while the clock signals include aclock enable signal CKE# and complementary clock signals CLK, CLK#, withthe “#” designating a signal as being active LOW. The command signalsRAS#, CAS#, and WE# are driven to values corresponding to a particularcommand, such as a read, write, or auto-refresh command.

In response to the clock signals CLK, CLK#, the command decoder 134latches and decodes an applied command, and generates a sequence ofinternal clock and control signals that control the components 102-132to execute the function of the applied command. The clock enable signalCKE enables clocking of the command decoder 134 by the clock signalsCLK, CLK#. The command decoder 134 further includes mode registers 136.Data written to the mode registers 136 are used to set various modes ofoperation, for example, burst data length, burst type, power-down mode,CAS latency, and the like. The command decoder 134 will generate theappropriate internal clock and control signals based on the modes set bythe data stored in the mode registers 136.

The memory device 100 further includes an address register 102 thatreceives row, column, and bank addresses over an address bus ADDR, witha memory controller (not shown) typically supplying the addresses. Theaddress register 102 receives a row address and a bank address that areapplied to a row address multiplexer 104 and bank control logic circuit106, respectively. The row address multiplexer 104 applies either therow address received from the address register 102 or a refresh rowaddress from a refresh counter 108 to a plurality of row address latchand decoders 110A-D. The bank control logic 106 activates the rowaddress latch and decoder 110A-D corresponding to either the bankaddress received from the address register 102 or a refresh bank addressfrom the refresh counter 108, and the activated row address latch anddecoder latches and decodes the received row address.

In response to the decoded row address, the activated row address latchand decoder 110A-D applies various signals to a corresponding memorybank 112A-D to thereby activate a row of memory cells corresponding tothe decoded row address. Each memory bank 112A-D includes a memory-cellarray having a plurality of memory cells arranged in rows and columns,and the data stored in the memory cells in the activated row is storedin sense amplifiers in the corresponding memory bank. The row addressmultiplexer 104 applies the refresh row address from the refresh counter108 to the decoders 110A-D and the bank control logic circuit 106 usesthe refresh bank address from the refresh counter when the memory device100 operates in an auto-refresh or self-refresh mode of operation inresponse to an auto- or self-refresh command being applied to the memorydevice 100, as will be appreciated by those skilled in the art.

A column address is applied on the ADDR bus after the row and bankaddresses, and the address register 102 applies the column address to acolumn address counter and latch 114 which, in turn, latches the columnaddress and applies the latched column address to a plurality of columndecoders 116A-D. The bank control logic 106 activates the column decoder116A-D corresponding to the received bank address, and the activatedcolumn decoder decodes the applied column address. Depending on theoperating mode of the memory device 100, the column address counter andlatch 114 either directly applies the latched column address to thedecoders 116A-D, or applies a sequence of column addresses to thedecoders starting at the column address provided by the address register102. In response to the column address from the counter and latch 114,the activated column decoder 116A-D applies decode and control signalsto an I/O gating and data masking circuit 118 which, in turn, accessesmemory cells corresponding to the decoded column address in theactivated row of memory cells in the memory bank 112A-D being accessed.

During data read operations, data being read from the addressed memorycells is coupled through the I/O gating and data masking circuit 118 toa read latch 120. The I/O gating and data masking circuit 118 supplies Nbits of data to the read latch 120, which then applies two N/4 bit wordsto a multiplexer 122. In the embodiment of FIG. 3, the circuit 118provides 32 bits to the read latch 120 which, in turn, provides four 8bits words to the multiplexer 122. A data driver 124 sequentiallyreceives the N/4 bit words from the multiplexer 122 and also receives adata strobe signal DQS from a strobe signal generator 126 and a delayedclock signal CLKDEL from the delay-locked loop 123. The DQS signal isused by an external circuit such as a memory controller (not shown) inlatching data from the memory device 100 during read operations. Inresponse to the delayed clock signal CLKDEL, the data driver 124sequentially outputs the received N/4 bits words as a corresponding dataword DQ, each data word being output in synchronism with a rising orfalling edge of a CLK signal that is applied to clock the memory device100. The data driver 124 also outputs the data strobe signal DQS havingrising and falling edges in synchronism with rising and falling edges ofthe CLK signal, respectively. Each data word DQ and the data strobesignal DQS collectively define a data bus DATA. The DATA bus alsoincludes masking signals DMO-M for masking write data of data writeoperations, as will be described in more detail below.

During data write operations, an external circuit such as a memorycontroller (not shown) applies N/4 bit data words DQ, the strobe signalDQS, and corresponding data masking signals DM on the data bus DATA. Adata receiver 128 receives each DQ word and the associated DM signals,and applies these signals to input registers 130 that are clocked by theDQS signal. In response to a rising edge of the DQS signal, the inputregisters 130 latch a first N/4 bit DQ word and the associated DMsignals, and in response to a falling edge of the DQS signal the inputregisters latch the second N/4 bit DQ word and associated DM signals.The input register 130 provides the two latched N/4 bit DQ words as anN-bit word to a write FIFO and driver 132, which clocks the applied DQword and DM signals into the write FIFO and driver in response to theDQS signal. The DQ word is clocked out of the write FIFO and driver 132in response to the CLK signal, and is applied to the I/O gating andmasking circuit 118. The I/O gating and masking circuit 118 transfersthe DQ word to the addressed memory cells in the accessed bank 112A-Dsubject to the DM signals, which may be used to selectively mask bits orgroups of bits in the DQ words (i.e., in the write data) being writtento the addressed memory cells.

As previously described, commands are issued to the memory device 100 inthe form of command signals, which are decoded by the command decoder134 to generate internal clock and control signals to perform therequested operation. FIG. 2 is a command decoding truth table for thememory device 100. The three command signals RAS#, CAS#, and WE# provideeight different commands for the memory device 100. These commandsinclude LOAD MODE, REFRESH, PRECHARGE, BANK ACTIVATE, WRITE, READ, NOP(no operation), and a RESERVED command, which can be used in the futurefor an additional command. The LOAD MODE command is used to load datainto the mode registers 136, which, as previously discussed, is used toset various modes of operation, for example, burst data length, bursttype, power-down mode, CAS latency, and the like. The REFRESH command isused to invoke a refresh sequence in the memory banks 112A-D. ThePRECHARGE command is used to deactivate or “close” activated, or “open,”memory banks 112A-D. The BANK ACTIVATE command is used to open at leastone of the memory banks 112A-D, as selected by a bank address, inpreparation for an memory access operation. The WRITE command and theREAD command are used to invoke a data write operation and a data readoperation, respectively, as previously described. The NOP operation isused to prevent unwanted commands from being registered during idle orwait states of the memory device 100.

The use of the RAS#, CAS#, and WE# signals provides an effective way toissue commands to the memory device 100. However, there are limitationsthat are inherent with this conventional approach. One such limitationis the maximum number of different possible commands that are providedwith the conventional command decoding of the memory device 100. Asillustrated above, due to the binary nature of the RAS#, CAS#, and WE#signals, the three command signals provide a maximum decoding of eightdifferent commands. Although eight different commands are sufficient forcurrent technology, it is easy to imagine that in the future there maybe the need for more than the eight commands previously described. Ifadditional commands are desired, additional command signals will need tobe used. For example, if two additional commands for a total of tencommands are desired, one command could be decoded using the RESERVEDcommand. However, the other additional command would require adding onemore signal to the existing three commands signals. With four commandsignals, there are now potentially sixteen different commands that canbe decoded.

Although having additional choices for commands appears to beadvantageous, it raises another limitation of the conventional commanddecoding employed by the memory device 100. That is, increasing thenumber of command signals to increase the number of different commandsthat can be decoded, will increase the number of external terminals or“pins” of the memory device 100 that are required to receive the commandsignals. In the present example, three external terminals are used bythe memory device 100 for receiving the RAS#, CAS#, and WE# signals.Having more than the eight possible commands requires at least fourexternal terminals to provide sufficient command decoding. The numbercan increase to twice that number in light of the development of systemsemploying differential command signals where each command signal isapplied to the memory device as a pair of complementary signalsrequiring two external terminals.

Currently, the number of external terminals that can be included with aconventionally packaged memory device is reaching its physical limits.The physical dimensions of the memory device package can always beincreased to accommodate additional external terminals, however, thissolution conflicts with the desirability of creating portable andcompact electronic systems. Additionally, adding external terminalsincreases the number of signal lines that must be used to communicatebetween a memory controller and the memory device. As applied to memorymodules, the additional external terminals of each memory device willrequire additional conductive traces to be formed on the differentlayers of the printed circuit board (“PCB”), thus, increasing thecomplexity of the PCB in design and manufacture. As a result, increasingthe number of external terminals in response to the need to accommodateadditional command choices is highly undesirable.

Therefore, there is a need for an alternative system and method forcommand decoding that can be used to provide greater flexibility inincreasing the number of commands and/or reducing the number of commandsignals used in decoding commands.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a system and methodfor decoding command signals. In one aspect, a command decoder isconfigured to generate internal control signals to perform an operationselected from a first set of operations according to the latched logiclevels of the command signals when the memory system is in a firstoperating state. The command decoder is further configured to generateinternal control signals to perform an operation selected from a secondset of operations according to the latched logic levels of the commandsignals when the memory system is in a second operating state. Inanother aspect, a method of decoding command signals includes receivingcommand signals, and selecting one operation from a first set ofoperations in accordance with the command signals while the memorysystem is in a first operating state, and selecting one operation from asecond set of operations in accordance with the command signals whilethe memory system is in a second operating state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a memory system havingconventional command decoding.

FIG. 2 is a truth table of conventional command decoding utilized in thememory system of FIG. 1.

FIG. 3 is a functional block diagram of a memory system having commanddecoding according to an embodiment of the present invention.

FIG. 4 is a truth table of the command decoding utilized in the memorysystem of FIG. 3 according to an embodiment of the present invention.

FIG. 5 is a state diagram for the command decoding shown in the truthtable of FIG. 4.

FIG. 6 is a truth table of command decoding that can be utilized in thememory system of FIG. 3 according to an alternative embodiment of thepresent invention.

FIG. 7 is a state diagram for command decoding that can be utilized inthe memory system of FIG. 3 according to an alternative embodiment ofthe present invention.

FIG. 8 is a functional block diagram of a processing system including amemory system having command decoding according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention provide systems and methods forcommand decoding in a memory that decodes commands based on commandsignals in combination with an operating state of the memory. Certaindetails are set forth below to provide a sufficient understanding of theinvention. However, it will be clear to one skilled in the art that theinvention may be practiced without these particular details. In otherinstances, well-known circuits, control signals, timing protocols, andsoftware operations have not been shown in detail in order to avoidunnecessarily obscuring the invention.

FIG. 3 illustrates a memory device 300 having control logic and commanddecoder 334 according to an embodiment of the present invention. Many ofthe circuits included in the memory device 300 have been previouslydescribed with respect to the memory device 100 of FIG. 1, andconsequently, in the interest of brevity, will not be described again.Operation of these circuits in the memory device 300 are the same as inpreviously described with respect to the memory device 100 of FIG. 1,and are referenced in FIG. 3 using the same reference number. Thecommand decoder 334, however, is different than the command decoder 134of the memory device 100. The command decoder 334 decodes commandsignals applied to the memory device 300 and generates internal clockand control signals to execute the requested command based on thecommand signals in combination with a operating state of the memorydevice 300. Thus, unlike the command decoder 134 of the memory device100, the command decoder 334 generates the internal signals to perform arequested operation based on the set of command signals applied to thememory device 300 and its current operating state. As a result, thenumber of command signals needed for decoding the commands can bereduced while maintaining the same number of different commandsavailable. Alternatively, the number of different commands can beincreased without increasing the number of command signals.

For example, the memory device 300 has the same number of commands aspreviously described for the memory device 100, and shown in the tableof FIG. 2. Namely, the eight different commands are LOAD MODE REGISTERS,REFRESH, PRECHARGE, BANK ACTIVATE, WRITE, READ, RESERVED, and NOP. Theseeight commands for the memory device 100 can be selected based on thecombination of the logic states of the three command signals RAS#, CAS#,and WE#. In contrast, and as will be described in more detail below, thememory device 300 can select one of the same eight commands using onlytwo command signals, RAS# and WE#, in combination with monitoring a bankactive state. In this particular embodiment, the command decoder 334 iscoupled to the bank control logic 306 to receive a bank active statesignal BACTIVE having a logic state indicative of whether any of thememory banks 112A-D are “open” (i.e., active) or if all of the banks are“closed” (i.e., precharged).

FIG. 4 illustrates a truth table for command decoding in the memorydevice 300 of FIG. 3 according to an embodiment of the presentinvention. Column 402 lists the commands for different combinations ofthe RAS# and WE# signals when any of the memory banks 312A-D are open.Column 404 lists the commands for different combinations of the RAS# andWE# signals for when all of the memory banks 312A-D are closed. Forexample, in the event the command decoder 334 receives a HIGH RAS#signal and a LOW WE# signal at the time the BACTIVE signal is active,indicating that at least one of the memory banks 312A-D is open, thecommand decoder 334 will generate internal clock and control signals toexecute a WRITE operation to at least one memory location in the memorybanks 312A-D identified by the address and bank signals, as previouslydescribed. However, if the HIGH RAS# signal and LOW WE# signal areapplied to the memory device 300 at a time the BACTIVE signal isinactive, indicating that all of the memory banks 312A-D are closed, thecommand decoder 334 will generate internal clock and control signals toexecute a LOAD MODE command that loads mode information into the moderegister 136, as previously described. As shown by comparing the truthtable of FIG. 4 with the truth table of FIG. 2, the memory device 300has the same eight commands as the memory device 100, but only requiresthe two RAS# and WE# signals instead of the three RAS#, CAS#, and WE#signals.

FIG. 5 is a state diagram illustrating the command decoding of thememory device 300 as previously described with respect to the commanddecoding truth table of FIG. 4. FIG. 5 illustrates various memory deviceoperations, represented by circles, and various memory device states,represented by shaded circles. The memory device 300 changes states andperforms operations in response to command signals RAS# and WE#, withcommand sequences (defined by a combination of command signals) andchanges of state represented by arrows pointing from a memory devicestate to a memory device operation. Bold arrows in FIG. 5 represent anautomatic sequence that returns the memory device 300 to a memory devicestate upon completion of the memory device operation. As will beexplained in more detail below, FIG. 5 illustrates command decoding on aper bank basis for memory banks 0 to N, which correspond to the memorybanks 112A-D of the memory device 300. Memory bank selection is made byway of bank addresses applied to the memory device at the time thecommand signals are latched.

Upon application of power to the memory device 300, a power-on sequenceis executed to place the memory device 300 in a POWER ON state 502. Whenthe power-on sequence is complete, the memory device 300 exits the POWERON state 502 and is reset to an ALL BANK PRECHARGED state 504. While inthe ALL BANK PRECHARGED state 504 and all of the memory banks 112A-D areclosed, application of the RAS# and WE# signals are be decoded by thecommand decoder 334 into one of three commands (not including theRESERVED command). The three commands shown in FIG. 5 are a REFRESHcommand that causes the memory device 300 to execute an AUTO REFRESHoperation 506, a LOAD MODE command that causes the memory device 300 toexecute an LOAD MRS operation 508, or a BANK ACTIVATE command thatcauses the memory device 300 to execute an ACTIVATE BANK command 509 fora particular memory bank 0 through N.

The REFRESH command is decoded from a LOW RAS# signal and a LOW WE#signal applied to the command decoder 334 while the memory device 300 isin the ALL BANK PRECHARGED state 504. In response, the memory device 300changes from the ALL BANK PRECHARGED state 504 to execute the AUTOREFRESH sequence 506. When the AUTO REFRESH sequence 506 is completed,the memory device 300 returns to the ALL BANK PRECHARGED state 504, asshown by the bold arrow pointing from the AUTO REFRESH state 506 back tothe ALL BANK PRECHARGED state 504, to await further commands. Similarly,the LOAD MODE command is decoded from a HIGH RAS# signal and a LOW WE#signal while the memory device 300 is in the ALL BANK PRECHARGED state504. The LOAD MODE command is shown in FIG. 5 as “WR” representing thecombination of a HIGH RAS# signal and LOW WE# signal, which would bedecoded to a WRITE command if the memory device was in the BANK ACTIVEstate 510. In response to the LOAD MODE command, the state of the memorydevice 300 changes from the ALL BANK PRECHARGED state 504 to execute theLOAD MRS operation 508, whereupon completion, the memory device 300returns to the ALL BANK PRECHARGED state 504.

The third command that can be decoded when the memory device 300 is inthe BANK CLOSED state 504 is the BANK ACTIVATE command, decoded from aLOW RAS# signal and a HIGH WE# signal. The BANK ACTIVATE command isshown in FIG. 5 as “ACT” representing the combination of a LOW RAS#signal and a HIGH WE# signal. As previously described, a BANK ACTIVATEcommand is a per bank command that activates at least one of the memorybanks 312A-D, typically identified by bank address signals. The BANKACTIVATE command causes the memory device 300 to change from the ALLBANK PRECHARGED state 504 to execute an ACTIVATE BANK operation 509 forthe particular memory bank. Upon completion of the ACTIVATE BANKoperation 509, the state of the memory device 300 for the particularmemory bank changes to a BANK ACTIVE state 510. The activated memorybank 312A-D remains activated until a PRECHARGE command is decoded bythe memory device 300, whereupon the state of the memory device 300changes to from the BANK ACTIVE state 510 to execute a PRECHARGE BANKoperation 511. Upon completing the PRECHARGE BANK operation 511, thememory device 300 returns through a BANK PRECHARGE state 505 to the ALLBANK PRECHARGED state 504. A BANK ACTIVATE command decoded while thememory device 300 is in the BANK PRECHARGE state 505 will execute theACTIVATE BANK operation 509 and change the state of the memory device300 to the BANK ACTIVE state 510. Two other commands that can be decodedin addition to the PRECHARGE command from the RAS# and WE# signals whilethe memory device 300 is in the BANK ACTIVE state 510 are a READ commandthat causes the memory device 300 to execute a READ operation 512 and aWRITE command that causes the memory device 300 to execute a WRITEoperation 514. A no operation NOP command can also be decoded by thecommand decoder 334 from the RAS# and WE# signals while the memorydevice 300 is in the BANK ACTIVE state 510, but as well known, typicallydoes not cause the memory device to perform an operation. As shown inFIG. 5, when a NOP command is decoded during the BANK ACTIVE state 510the memory device does not change states. When the READ and WRITEcommands are decoded, a respective command is executed and uponcompletion, the memory device 300 returns to the BANK ACTIVE state 510.

As illustrated by the state diagram 500, the same specific combinationof the RAS# and WE# signals can be decoded by the command decoder 334 toexecute different sets of commands depending on the particular operatingstate of the memory device 300. For example, the combination of a LOWRAS# signal and a LOW WE# signal is labeled in the state diagram 500 as“PRE.” In one case where the memory device is in the ALL BANK PRECHARGEDstate 504, the PRE combination of the RAS# and WE# signals results inthe memory device 300 changing from the ALL BANK PRECHARGED state 504 toexecute the AUTO REFRESH operation 506 to perform an auto-refreshsequence. However, if the PRE combination of RAS# and WE# signals isapplied when the memory device 300 is in the BANK ACTIVE state 510, itcauses the memory device 300 to change from the BANK ACTIVE state 510 tothe BANK PRECHARGED state 505 by executing the PRECHARGE BANK operation511. Thus, using just the one PRE combination of RAS# and WE# signals,two different commands can be decoded.

FIG. 6 illustrates a command decoding truth table 600 for the memorydevice 300 according to an alternative embodiment of the presentinvention. The command decoding truth 600 illustrates the use of aninput signal that is not typically used as a command signal to provideadditional command decoding choices. In the embodiment that will bedescribed with reference to FIG. 6, the address signal A10 is used, incombination with the RAS# and WE# command signals shown in column 602,and further in combination with the memory bank open/closed states shownin column 604, to select a particular refresh or precharge mechanismshown in column 606. Conventional memory devices have utilized a signaltypically not used for commands, such as the A10 signal, to selectdifferent refresh and precharge mechanisms. However, conventionaldevices do not utilize command decoding as previously describedaccording to an embodiment of the present invention. Thus, thecombination of the command decoding of embodiments of the presentinvention with the use of a signal not typically used for commands, suchas the A10 signal, provides additional alternative embodiments of thepresent invention.

Operation of the command decoding shown in the truth table 600 will nowbe described with respect to the memory device 300. When the memorydevice 300 is in a BANK CLOSED state, and a LOW RAS# signal and a LOWWE# signal are applied to the memory decoder 334, a refresh command isdecoded. The logic level of the A10 signal is used to select whether therefresh sequence is executed on a per memory bank basis, or all memorybanks 312A-D are refreshed concurrently. As shown in columns 606 and608, a LOW A10 signal selects a per memory bank refresh operation and aHIGH A10 signal selects an all memory bank refresh operation. As furthershown in column 608, the A10 signal is interpreted as part of a validvalue when a BANK ACTIVATE command or a LOAD MODE command is issuedwhile the memory device 300 is in the BANK CLOSED state.

As previously described, the A10 signal is used to select a refreshsequence when a refresh command is issued during the BANK CLOSED state.Additionally, the A10 signal is used to select various prechargesequences when a precharge command is issued during a BANK OPEN state.As shown in column 608, the A10 signal is used in combination with theRAS# and WE# signals to select single or all bank precharge,auto-precharge following a write command, and auto-precharge following aread command. The A10 signal can be used to select whether anauto-precharge sequence is executed following a write or read commandsince the A10 signal is not used to select a particular memory locationfor access.

FIG. 7 illustrates a state diagram 700 for command decoding by thememory device 300 according to another embodiment of the presentinvention. The state diagram 700 of FIG. 7 is similar to the statediagram 500 of FIG. 5 and uses the same convention as previouslydescribed with respect to the memory device state, memory deviceoperation, command sequence and automatic sequences. Also as with thestate diagram 500 of FIG. 5, the command decoding can be on a per memorybank basis for memory banks 0 through N. The particular memory bank isselected by the bank address signals provided to the memory device 300at the time the command signals RAS# and WE# are latched. In comparisonto the state diagram 500 of FIG. 5, additional commands and operatingstates are defined in the state diagram 700 without increasing thenumber of command signals. All of the commands and states shown in thestate diagram 700 of FIG. 7 can be decoded based on the RAS# and WE#signals, in combination with the particular operating state of thememory device 300.

An additional state shown in the state diagram 700 is a POWER DOWN state702. The POWER down state 702 represents a condition where the memorydevice 300 is placed into a low-power state during which many of thecircuits are deactivated, such as the output drivers 124 (FIG. 3), theinput receivers 128, and the DLL 123 in order to reduce powerconsumption. The POWER DOWN state 702 can be reached by applying a HIGHRAS# signal and a HIGH WE# signal from the ALL BANK PRECHARGED state504. In contrast to the command decoding illustrated in the truth table400 of FIG. 4, the state diagram 700 utilizes the RESERVED command tochange to the POWER DOWN state 702. The RESERVED command is shown inFIG. 7 as “RD” representing the combination of a HIGH RAS# signal and aHIGH WE# signal, which is the combination of the command signals for aREAD command in a BANK ACTIVE state 510. From the POWER DOWN state 702,the state of the memory device 300 can be changed to a SLEEP POWER DOWNstate 704 or a SELF REFRESH state 706 depending on the RAS# and WE#signals applied. Where a HIGH RAS# signal and a HIGH WE# signal areapplied to the command decoder 334 when the memory device 300 is in thePOWER DOWN state 702, the operating state changes to the SLEEP POWERDOWN state 704. The SLEEP POWER DOWN state 704 represents a conditionwhere the memory device 300 is placed into a low-power state to agreater extent than in the POWER DOWN state 702. For example, the SLEEPPOWER DOWN state 704 may not require a CLK signal to be applied, andconsequently, clock buffers of the memory device 300 can be deactivatedin addition to the input and output circuits. To return from the SLEEPPOWER DOWN state 704 to the POWER DOWN state 702, a LOW RAS# signal anda LOW WE# signal are applied to the command decoder 334. For the memorydevice 300 to change from the POWER DOWN state 702 to the SELF REFRESHstate 706, a LOW RAS# signal and a LOW WE# signal are applied to thecommand decoder 334. During the SELF REFRESH STATE 706, the memorydevice 300 executes a self-refresh sequence to refresh memory cells ofthe memory banks 312A-D. Upon completion of the self-refresh sequence,the state of the memory device 300 returns to the POWER DOWN state 702.

As illustrated in FIG. 7, for a particular combination of the RAS# andWE# signals, an even greater number of commands can be decoded byapplying the specific combination of RAS# and CAS# signals duringdifferent operating states of the memory device 300. For example, takingthe combination of a LOW RAS# signal and a LOW WE# signal, labeled inthe state diagram 700 as “PRE,” four different commands can be decodedfrom the same PRE combination. The first command, which results from theapplication of the PRE combination of RAS# and WE# signals during theALL BANK PRECHARGED state 504, causes the memory device 300 to changefrom the ALL BANK PRECHARGED state 504 to execute the AUTO REFRESHoperation 506. The second command, which results from application of thePRE combination of the RAS# and WE# signals during the BANK ACTIVE state510, causes the memory device 300 to execute the PRECHARGE BANK command511 to change to the BANK PRECHARGED state 505. The third command, whichresults from application of the PRE combination of the RAS# and WE#signals during the POWER DOWN state 702, causes the memory device 300 tochange to the SELF REFRESH state 706 to execute a self-refresh sequence.The fourth command, which results from the application of the PREcombination of the RAS# and WE# signals during the SLEEP POWER DOWNstate 704 causes the memory device 300 to exit the SLEEP POWER DOWNstate 704 to the POWER DOWN state 702. Thus, four different commands canbe decoded using only one combination of the two RAS# and WE# commandsignals, in combination with the memory state of the memory device 300.

FIG. 8 is a block diagram of a processing system 800 includingprocessing circuitry 802 including the memory device 300 having acommand decoder 334 that utilizes command decoding according to anembodiment of the present invention. Typically, the processing circuitry802 is coupled through address, data, and control buses to the memorydevice 300 to provide for writing data to and reading data from thememory device. The processing circuitry 802 includes circuitry forperforming various processing functions, such as executing specificsoftware to perform specific calculations or tasks. In addition, theprocessing system 800 includes one or more input devices 804, such as akeyboard or a mouse, coupled to the processing circuitry 802 to allow anoperator to interface with the processing system 800. Typically, theprocessing system 800 also includes one or more output devices 806coupled to the processing circuitry 802, such as output devicestypically including a printer and a video terminal. One or more datastorage devices 808 are also typically coupled to the processingcircuitry 802 to store data or retrieve data from external storage media(not shown). Examples of typical storage devices 808 include hard andfloppy disks, tape cassettes, compact disk read-only (CD-ROMs) andcompact disk read-write (CD-RW) memories, and digital video disks(DVDs).

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. For example, the embodiments ofthe invention described herein have been described with respect tomonitoring the open or closed state of the banks of memory 112A-D as anexample of an operating state that is monitored for decoding the commandsignals. However, other embodiments of the invention monitor alternativeoperating states in decoding the command signals, including thedifferent operating states previously described, such as a power downstate and refresh states. Accordingly, the invention is not limitedexcept as by the appended claims.

What is claimed is:
 1. An apparatus, comprising: a latch configured toreceive and latch a command; and a command decoder coupled to the latchand configured to receive the command and receive a bank signalindicative of a state of a bank, the command decoder configured toperform a first operation responsive, at least in part, to a firstcommand and the bank signal indicating the bank is in a first state andfurther configured to perform a second operation responsive, at least inpart, to the first command and the bank signal indicating the bank is ina second state, the command decoder further configured to perform athird operation responsive, at least in part, to a second command andthe bank signal indicating the bank is in the first state.
 2. Theapparatus of claim 1, further comprising: bank control logic configuredto provide the bank signal to the command decoder; and wherein the firststate corresponds to a first operating state of the bank and the secondstate corresponds to a second operating state of the bank.
 3. Theapparatus of claim 1, wherein the first operation is one of plurality ofbank active operations and the second operation is one of plurality ofbank inactive operations.
 4. The apparatus of claim 3, wherein at leastone of the plurality of bank active operations is configured to switchthe bank to switch to a third state.
 5. The apparatus of claim 1,wherein the command comprises at least one of a row address selectsignal, a write enable signal, or an A10 signal.
 6. The apparatus ofclaim 1, wherein the apparatus is included in a memory.
 7. A method ofdecoding command signals applied to a memory bank, comprising:monitoring an operating state of the memory bank, the memory bank havinga precharged operating state and an active operating state; performingan operation from a first set of operations in accordance with a firstcombination of the command signals responsive, at least in part, to thememory bank is operating in the precharged operating state; andperforming an operation from a second set of operations in accordancewith second combination of the command signals responsive, at least inpart, to the memory bank is operating in the active second operatingstate.
 8. The method of claim 7, further comprising: performing anoperation from a third set of operations in accordance with the commandsignals while the memory bank is in a third operating state.
 9. Themethod of claim 8, further comprising: monitoring a logic level of asignal applied to the memory bank.
 10. The method of claim 9, whereinsaid monitoring a logic level of a signal applied to the memory bankcomprises: monitoring the logic state of an address signal.
 11. Themethod of claim 8, wherein said performing an operation from the firstset of operations in accordance with the command signals comprises:performing an operation that results in the memory bank changing fromthe precharged operating state to the third operating state.
 12. Themethod of claim 11, wherein the thud operating state comprises a powerdown state.
 13. The method of claim 7, wherein said performing anoperation from a first set of operations in accordance with the commandsignals while the memory bank is in the precharged operating statecomprises: performing a first mode of an operation in response to thesignal having a first logic level and performing a second mode of theoperation in response to the signal having the second logic level. 14.The method of claim 7, wherein said monitoring an operating state of thememory bank comprises: receiving a bank state signal; and determiningthe operating state of the memory bank based, at least in part, on thebank state signal.
 15. A method, comprising: receiving a command;receiving a bank state signal indicative of a state of a memory bank;performing a first operation based, at least in part, on a first commandand the bank state signal indicating the bank has a first state;performing as second operation based, at least in part, on the firstcommand and the bank state signal indicating the bank has a secondstate; and performing a third operation based least in part, on a secondcommand and the bank state signal indicating the bank has the firststate.
 16. The method of claim 15, further comprising: performing athird operation based, at least in part, on the bank state signalindicating the memory bank has a third state.
 17. The method of claim14, wherein said performing the first operation based, at least in part,on the bank state signal indicating the memory bank has a first statecomprises: performing an operation that causes the memory bank to changefrom the first state to a third state.
 18. The method of claim 14,wherein the first state corresponds to an active state and the secondstate corresponds to an inactive state.
 19. The method of claim 14,wherein said performing the first operation based, at least in part, onthe bank state signal indicating the bank has a first state comprises:generating a set of internal signals to perform the first operation.